Third-Generation (3G) mobile systems, such as for instance Universal Mobile Telecommunications System (UMTS) standardized within the Third-Generation Partnership Project (3GPP), have been based on Wideband Code Division Multiple Access (WCDMA) radio access technology. Today, the 3G systems are being deployed on a broad scale all around the world. A first step in enhancing this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), both of them providing an improvement of radio access in spectral efficiency and flexibility compared to plain UMTS.
While HSDPA and HSUPA still take the advantage of the WCDMA radio access technology, the next major step or evolution of the UMTS standard has brought a combination of Orthogonal Frequency Division Multiplexing (OFDM) for the downlink and Single Carrier Frequency Division Multiplexing Access (SC-FDMA) for the uplink. The new study item which has become later a work item has been named “Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS terrestrial Radio Access Network (UTRAN)”, abbreviated to E-UTRA and E-UTRAN and often referred to as Long-Term Evolution (LTE) since it is intended to cope with future technology evolutions.
The target of LTE is to achieve significantly higher data rates compared to HSDPA and HSUPA, to improve the coverage for the high data rates, to significantly reduce latency in the user plane in order to improve the performance of higher layer protocols (for example, TCP), as well as to reduce delay associated with control plane procedures such as, for instance, session setup. Focus has been given to the convergence towards use of Internet Protocol (IP) as a basis for all future services, and, consequently, on the enhancements to the packet-switched (PS) domain.
A radio access network is, in general, responsible for handling all radio-access related functionality including scheduling of radio channel resources. The core network may be responsible for routing calls and data connections to external networks. In general, today's mobile communication systems (for instance GSM, UMTS, cdma200, IS-95, and their evolved versions) use time and/or frequency and/or codes and/or antenna radiation pattern to define physical resources. These resources can be allocated for a transmission for either a single user or divided to a plurality of users. For instance, the transmission time can be subdivided into time periods usually called time slots then may be assigned to different users or for a transmission of data of a single user. The frequency band of such a mobile systems may be subdivided into multiple subbands. The data may be spread using a (quasi) orthogonal spreading code, wherein different data spread by different codes may be transmitted using, for instance, the same frequency and/or time. Another possibility is to use different radiation patterns of the transmitting antenna in order to form beams for transmission of different data on the same frequency, at the same time and/or using the same code.
The architecture defined in LTE is called Evolved Packet System (EPS) and comprises apart from E-UTRAN on the radio access side also the Evolved Packed Core (EPC) on the core network side. LTE is designed to meet the carrier needs for high-speed data and media transport as well as providing high capacity voice support to the next decade.
The LTE network is a two-node architecture consisting of access gateways (aGW) and enhanced base stations, so-called eNode Bs (eNB). The access gateways handle core network functions, i.e. routing calls and data connections to external networks, and also implement radio access network functions. Thus, the access gateway may be considered as combining the functions performed by Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN) in today's 3G networks and radio access network functions, such as for example header compression, ciphering/integrity protection. The eNodeBs handle functions such as for example Radio Resource Control (RRC), segmentation/concatenation, scheduling and allocation of resources, multiplexing and physical layer functions. E-UTRAN air (radio) Interface is thus an interface between a User Equipment (UE) and an eNodeB. Here, the user equipment may be, for instance, a mobile terminal, a PDA, a portable PC, a PC, or any other apparatus with receiver/transmitter conform to the LTE standard. The described architecture is exemplified in FIG. 19.
Multi carrier transmission introduced on the E-UTRAN air interface increases the overall transmission bandwidth, without suffering from increased signal corruption due to radio-channel frequency selectivity. The proposed E-UTRAN system uses OFDM for the downlink and SC-FDMA for the uplink and employs MIMO with up to four antennas per station. Instead of transmitting a single wideband signal such as in earlier UMTS releases, multiple narrow-band signals referred to as “subcarriers” are frequency multiplexed and jointly transmitted over the radio link. This enables E-UTRA to be much more flexible and efficient with respect to spectrum utilization.
In 3GPP LTE, the following downlink physical channels are defined (3GPP TS 36.211 “Physical Channels and Modulations”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference):                Physical Downlink Shared Channel (PDSCH)        Physical Downlink Control Channel (PDCCH)        Physical Broadcast Channel (PBCH)        Physical Multicast Channel (PMCH)        Physical Control Format Indicator Channel (PCFICH)        Physical HARQ Indicator Channel (PHICH)        
The PDSCH is utilised for data and multimedia transport and hence designed for high data rates. The PDSCH is designed for the downlink transport, i.e. from eNode B to at least one UE. In general, this physical channel is separated into discrete physical resource blocks and may be shared by a plurality of UEs. The scheduler in eNodeB is responsible for allocation of the corresponding resources, the allocation information is signalised. The PDCCH conveys the UE specific control information.
The general baseband signal processing in LTE is shown in FIG. 1 (3GPP TS 36.211 “Multiplexing and Channel Coding”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference). First, information bits which contain the user data or the control data are block-wise encoded (channel coding by a forward error correction such as turbo coding). The blocks of encoded bits are then scrambled 110. By applying different scrambling sequences for neighbouring cells in downlink, the interfering signals are randomized, ensuring full utilisation of the processing gain provided by the channel code. The blocks of scrambled bits, which form symbols of predefined number of bits depending on the modulation scheme employed, are transformed 120 to blocks of complex modulation symbols using the data modulator. The set of modulation schemes supported by LTE downlink includes QPSK, 16-QAM and 64-QAM corresponding to two, four or six bits per modulation symbol
Layer mapping 130 and precoding 140 are related to Multiple-Input/Multiple-Output (MIMO) applications supporting more receiving and/or transmitting antennas. LTE supports up to four transmitting antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. The set of resulting symbols to be transmitted on each antenna is further mapped 150 on the resources of the radio channel, i.e., into the set of resource blocks assigned for particular UE by a scheduler for transmission. The selection of the set of resource blocks by the scheduler depends on the channel quality indicator (CQI)—a feedback information signalized in the uplink by the UE and reflecting the measured channel quality in the downlink. After mapping of symbols into the set of physical resource blocks, an OFDM signal is generated 160 and transmitted. The generation of OFDM signal is performed using inverse discrete Fourier transformation.
The physical resources for the OFDM transmission are often illustrated in a time-frequency grid wherein each column corresponds to one OFDM symbol and each row corresponds to one OFDM subcarrier, the numbering of columns thus specifying the position of resources within the time domain, and the numbering of the rows specifying the position of resources within the frequency domain.
FIG. 2 illustrates the time domain structure for LTE transmission. The radio frame 230 has a length of Tframe=10 ms, corresponding to the length of a radio frame in previous UMTS releases. Each radio frame further consists of ten equally sized subframes 220 of the equal length Tsubframe=1 ms. Each subframe 220 further consists of two equally sized time slots (TS) 210 of length Tslot=0.5 ms. Each slot finally consists of a number of OFDM symbols including a cyclic prefix of predefined length. Here, the OFDM symbol refers to a symbol to be transmitted, being formed by the inverse discrete Fourier transformation of a column in the resource grid, consisting of subcarrier symbols to be transmitted within one time interval. Prefix of an OFDM symbol has a function of separating the OFDM symbols in order to cope with the inter-symbol interference. LTE standard defines cyclic prefixes with two different lengths, a normal cyclic prefix and an extended cyclic prefix. According to the length of the prefix, for the subcarrier spacing of 15 kHz there are either seven or six OFDM symbols per slot, respectively.
The time-frequency grid of subcarriers and OFDM symbols for one time slot TS0 210 in downlink is illustrated in FIG. 3. A smallest time-frequency resource corresponding to a single subcarrier of an OFDM symbol is referred to as a resource element 310. The downlink subcarriers are further grouped into physical resource blocks (PRB) 320. Each physical resource block 320 consists of twelve consecutive subcarriers which form a so-called subband and span over the 0.5 ms slot 210 with the specified number of OFDM symbols. Such subband occupies a bandwidth of 180 kHz.
In order to estimate the downlink channel in case of the OFDM transmission, reference signals (pilots) are regularly inserted into the time-frequency grid. These symbols are referred to as LTE downlink reference signals. FIG. 4 illustrates the distribution of the LTE downlink reference signals 401. Hereby, FIG. 4A shows the LTE downlink reference signals 401 distributions within a subframe for one antenna port 410. FIG. 4B and FIG. 4C show the LTE downlink reference signals 401 for two antenna ports 421, 422, and for four antenna ports 441, 442, 443, 444, respectively. For more than one antenna port, the resource elements 402, the position of which corresponds to the positions of LTE downlink reference signals at another antenna port(s), are not used for transmission. This is because a UE needs to get an accurate Carrier to Interference Ratio (CIR) estimation for each transmitting antenna. Hence when a reference signal is transmitted from one antenna port, the other antenna ports in the cell are idle.
Each LTE resource block consists of 12 subcarriers and a predefined number of OFDM symbols, for instance seven or six, which results in an amount of resource elements (in this example case 84 or 72, respectively). However, from this amount of resource elements only a subset may be used for the transmission of the data. Some resources are reserved for the LTE downlink reference signals as can be seen in FIG. 4. Another portion is used for layer 1 (L1) and layer 2 (L2) control signalling.
In LTE, the L1/L2 control signals are mapped to the first n OFDM symbols of a subframe, wherein n is more than or equal to 1 and is less than or equal to three. Transmitting of the L1/L2 control signals in the beginning of the subframe has the advantage of early decoding of the corresponding L1/L2 control information included therein. Thus, there is no mixing of control signalling and data within an OFDM symbol. Consequently, the subframe 220 consists of two time slots TS0 and TS1. The first time slot TS0 carries both control and data OFDM symbols. The second time slot TS1 then only carries the data symbols. Hence the number of resource elements available for data in the first time slot TS0 depends on the number of control OFDM symbols. Furthermore, the number of resource elements available for data in both first and second time slots depends on the number of LTE downlink reference signals.
The resource mapping 150 of modulation symbols onto the resources of the radio channel after modulation mapping 120 and possibly layer mapping 130 and precoding 140, is performed by first forming a Virtual Resource Block (VRB). For each antenna port used for transmission of a physical channel, the block of complex valued symbols is mapped to resource elements first in the order of frequency (filling rows of a column) and then in the time order (filling columns) as can be seen in FIG. 5. A pair of virtual resource blocks represents the smallest resource portion that can be addressed by the scheduler. It has the size and structure of a subframe including the first and the second time slot. A pair of virtual resource blocks is then mapped onto a pair of physical resource blocks.
The following two types of virtual resource blocks are defined according to their mapping on the physical resource blocks in LTE downlink:                Localised Virtual Resource Block (LVRB)        Distributed Virtual Resource Block (DVRB)        
In the localised transmission mode using the localised VRBs, adjacent physical resource blocks are assigned for the transmission to a single user equipment. In the distributed transmission mode using the distributed VRBs, the physical resource blocks carrying data for the same UE are distributed across the frequency band.
In order to exploit multi-user diversity and to achieve scheduling gain in frequency domain, the data for a given user should be allocated on resource blocks on which the users have a good channel conditions. Typically, those resource blocks are close to each other and therefore, the localised mode is preferred. The pairs of VRBs of the localised type are mapped directly to the pairs of PRBs.
The distributed VRBs are adapted to utilise diversity gain when frequency selective scheduling is not appropriate for UEs. In order to maximise the diversity level of allocating multiple contiguous distributed VRBs to a single UE, the distributed VRBs are mapped on well-separated physical resource blocks.
For both types of VRBs pairs—localized and distributed—a subframe is addressed together by a single VRB number as shown in FIG. 5. VRB pairs are numbered from 0 to the number of allocated downlink resource blocks minus one. The desired frequency gap varies and is predefined depending on the system bandwidth (3GPP TS 36.213 “Physical Layer Procedures”, Release 8, v. 8.3.0, May 2008, available at http://www.3gpp.org and incorporated herein by reference). FIG. 5 shows an example of mapping of pairs of distributed virtual resource blocks into the pairs of physical resource blocks for a 5 MHz system bandwidth. For instance, in case of a 5 MHz LTE system, the system bandwidth consists of 24 PRBs in frequency. The frequency gap Ngap between the pair of the physical resource blocks on which a distributed virtual resource block pair is mapped is 12 physical resource blocks as can be seen in the PRBs grid 500. This frequency gap provides sufficient frequency diversity. In this example, two pairs VRB0 and VRB1 of VRBs have been allocated for a transmission to a UE. The first time slot TS0 of VRB0 is mapped on the first time slot TS0 of the first PRB in 500. The second time slot TS1 of the VRB0 is then mapped on the second time slot of the 13th PRB. Similarly, the first time slot of the VRB1 is mapped on the first time slot of the seventh PRB and the second time slot of the VRB1 is mapped on the second time slot of the 19th PRB. In this way, the frequency gap Ngap of 12 is achieved between the PRBs belonging to a single pair of VRBs. Moreover, the time gap of one time slot corresponding to seven OFDM symbols is achieved. In other words, a distributed VRB pair is hopped at half the system bandwidth. This scheme improves frequency diversity especially for larger system bandwidths.
The situation becomes more complicated if data symbols with repetition are mapped on the physical resources. This is illustrated in FIG. 6. Within a modulation, with symbol repetition of order two, the original data symbols 610 are repeated once obtaining the repeated data symbols 620. The information bits are mapped on to two constellations to obtain original data symbols and repeated data symbols The two constellations could be same or different. Thus, the repeated data symbol does not necessarily result in the same modulation symbol as the original data symbol. According to the above described mapping, the control symbols 630 fill the first n OFDM symbols of the first time slot of the VRB, followed by the original data 610 and the repeated data 620 in the first and the second time slot. The mapping into the physical resource blocks is then performed as described above, namely, the control symbols 630 together with a first portion of the original data symbols 611 are mapped to the first time slot of a first physical resource block PRB0. The rest of the original data symbols 612 is then together with the repeated symbols 620 mapped into the second time slot of a second physical resource block PRB12 with frequency gap of 12 between PRB0 and PRB12. However, due to the mapping of control symbols onto the first OFDM symbols of the first time slot of the VRB, the second time slot mapped on the same PRB (in frequency) now contains both, the original symbols 612 and their repetition. Thus, the desired level of diversity corresponding to the frequency gap of 12 is not achieved for the original and the repeated data symbols. The following Table shows the degree of achieving the desired frequency gap between the original and the repeated symbols with respect to this resource-mapping rule for a 5 MHz system:
PRB allocation size12345678910Frequency 84.0578.2063.7058.0056.00close to or less than 50diversity [%]
The degree of achieving the desired frequency gap of 12 between each pair of original and repeated data symbols has been obtained for all symbol pairs. The level of frequency diversity decreases with the increasing allocation size. Here, the allocation size refers to a number of PRB pairs allocated for the transmission. The time diversity in terms of distance in OFDM symbols between the data symbols also depends on the allocation size. The reduced diversity level results in the reduction of the system performance in terms of BLock Error Rate (BLER).